1. Technical Field
This invention relates to elastic power transmission belts and, more particularly, to an elastic composite power transmission belt that benefits from the characteristics of both components.
2. Background Art
Light duty power elastic transmission belts have been used for years to drive sewing machines and other small commercial machines. These have often included coil or garter springs, as shown, for example, in U.S. Pat. No. 3,482,462xe2x80x94Dahlem. In many applications the friction between the bare metal spring belt and the pulleys (and/or rollers) can excessively abrade both the belt and pulleys. In the transmission of power between rollers in light duty applications, urethane is usually well suited because of its excellent physical properties, which include elastic memory, flexibility and abrasive resistance. The primary drawback to the use of urethane is its inability to operate in heat, since its elastic memory declines as temperature rises. Compared to operation at 70xc2x0 F., its resilience drops about 30% at 125xc2x0 F., and up to 80% at 150xc2x0 F. This drastically reduces its life at elevated temperature to a matter of days, rather than years. Thus urethane, although excellent at lower temperatures, is unsuited for use at higher temperatures.
Another drawback of urethane is its buildup of static electricity when it rubs against metal pulleys or rollers, due to urethane being a good insulator. This makes urethane belts unsuitable for use in many electronic, paper handling and clean room applications. Attempts have been made to add metal or carbon fillers to make it conductive, but these additives change the physical properties and can emit damaging conductive dust when the belts are abraded during operation.
A recent innovation in power transmission belts is the use of static dissipative elastic urethane, although its utility is limited by its resistivity, which is insufficient for many applications. It has been proposed to reinforce urethane belts with embedded conductive metal cables, but, since the cables stretch very little, the resulting belts are nearly inelastic, making them unsuitable for many applications. Such a reinforced belt is shown in U.S. Pat. No. 2,288,669xe2x80x94Atkinson, which shows a coil spring imbedded in a rubber and braided fabric encasement. In these belts, the composite is forced to act as a monolithic unit, as governed by the relative inelasticity of the rubber and braided encasement. The wire core acts as a reinforcement only.
Thus, urethane belts are commonly used at room temperatures, but are unsuited to operation at elevated temperatures or in applications that must be free of static electricity. Materials like silicone and rubber belts can stand up to high temperatures and can be filled with carbon to reduce static electricity, but they abrade easily and have insufficient resilience and consequent short life. Reinforced belts are too inelastic. Thus most heat resistant belts are made of metal coil springs and operate well at elevated temperatures, but have shortened lives due to frictional wear (abrasion).
Wire coil spring and elastomeric composites have also been used as seals, as shown in U.S. Pat. No. 5,160,122xe2x80x94Balsells. Here again, the coil spring is embedded in the elastomer by molding and can take shapes other than cylindrical.
It would be desirable to provide a urethane power transmission belt reinforced by a heat resistant conductive spring. Such a composite belt is not presently available in commerce.
It is therefore an object of this invention to provide a urethane power transmission belt reinforced by a heat resistant conductive spring.
To produce such a belt, several obstacles must be overcome. There is a difficulty in simultaneously joining the ends of metal and plastic components together. Further, the differences in elasticity of metal and plastic materials in the same belt would under/overstretch one component if the stretch of the other component is optimized in a design. Urethane operates best when stretched 10%, but metal spring belts provide optimal operation when stretched about 30%.
Applicant has discovered how to produce a composite plastic and metal spring composite belt having optimal performance. This is done by providing an inner coil spring member slidable within the bore of a plastic tubular outer member, and precompressing the plastic member, while pretensioning the spring member in the uninstalled condition of the composite belt. When stretched upon installation, the stretching of the belt further tensions the spring member, and partly or completely decompresses the plastic member, resulting in little or no compression. This results in optimal tensioning of the spring member when installed over adjacent rollers or pulleys in operating condition, and assures no or little gap between the ends of the outer plastic member. Thus, the spring member is the driving member, while the tubular plastic member acts as a sheath, providing a composite belt having the desirable qualities of the plastic and the steel spring.
In one aspect this invention features a composite endless power transmission belt, comprising a resilient plastic outer tubular member having a cylindrical bore, and an inner coil spring member extending wholly and slidably within the cylindrical bore and endlessly through the bore, such that, when assembled but uninstalled, the outer tubular member is in compression and the inner coil spring member is in tension. Preferably, the inner and outer members are dimensioned so that, in installed condition, the spring member is in optimal operating tension.
In another aspect, this invention features a method of making a composite power transmission belt having an outer resilient plastic tubular member and an endless coil spring inner member slidingly received in the bore of the tubular outer member, comprising the steps of providing the outer member with a predetermined length, placing the inner member slidably within a bore of the outer member, and joining the ends of the inner member together to form an endless coil spring to compress the outer member and tension the inner member. Preferably, the inner and outer members are initially dimensioned so that, in installed condition, the outer member is uncompressed and the inner member is in optimal operating tension.